The present invention relates to a variable venturi-type carburetor.
A variable venturi-type carburetor normally comprises a suction piston which changes the cross-sectional area of the venturi portion of the carburetor in response to a change in the amount of air fed into the cylinder of the engine, a needle connected to the suction piston, a fuel passage extending in the axial direction of the needle so that the needle is able to enter into the fuel passage, and a metering jet arranged in the fuel passage and cooperating with the needle. In addition, there is known a variable venturi-type carburetor in which the air-fuel ratio of the mixture fed into the cylinder of the engine is controlled so that it is equal to the stoichiometric air-fuel ratio. This variable venturi-type carburetor comprises an air-bleed passage connected to the fuel passage and an electromagnetic control valve arranged in the air-bleed passage and actuated in response to an output signal from an oxygen concentration detector arranged in the exhaust passage of the engine. In this variable venturi-type carburetor, the air-fuel ratio of the mixture fed into the cylinder of the engine is made equal to the stoichiometric air-fuel ratio by gradually decreasing the amount of air fed into the fuel passage from the air-bleed passage when the air-fuel ratio of the mixture becomes greater than the stoichiometric air-fuel ratio and by gradually increasing the amount of air fed into the fuel passage from the air-bleed passage when the air-fuel ratio of the mixture becomes less than the stoichiometric air-fuel ratio. However, in a case where the air-bleed passage is connected to the fuel passage, the range of fluctuation of the air-fuel ratio, which fluctuation occurs when the flow area of the air-bleed passage varies by a certain degree, varies in accordance with the amount of fuel flowing within the fuel passage. That is, when the amount of air fed into the cylinder of the engine is large, that is, when the amount of fuel flowing within the fuel passage is large, the air-fuel ratio is not changed very much even if the flow area of the air-bleed passage is considerably changed. However, when the amount of air fed into the cylinder of the engine is small, that is, when the amount of fuel flowing within the fuel passage is small, if the flow area of the air-bleed passage is slightly changed, the air-fuel ratio is considerably changed. Consequently, in the conventional carburetor, in a case where the amount of fuel flowing within the fuel passage is small, when the air-fuel ratio becomes greater than the stoichiometric air-fuel ratio, and thus the amount of air fed into the fuel passage from the air-bleed passage is gradually decreased, the air-fuel ratio becomes considerably small. This results in a problem in that the air-fuel ratio considerably fluctuates.